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Developer guide to bzrlib transports
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This guide describes the `Transport` classes that Bazaar uses for most
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local and remote file access. (Working tree files are the major
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exception (`bug 606249 <https://bugs.launchpad.net/bzr/+bug/606249>`).
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A symlink creates an alias where files or directories can be accessed by a
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different name. Symlinks are useful but raise a few annoying cases for
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It's important to have tests for symlinks but these tests can't run on
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Windows, so you need eg ::
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_test_needs_features = [tests.SymlinkFeature]
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self.requireFeature(tests.SymlinkFeature)
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Bazaar versions symlinks as objects in their own right, whose content is
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the path they point to. bzr doesn't care whether a versioned
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symlink is absolute or relative; or whether it points inside or outside
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the working tree; or whether its referent exists or not. In Unix the
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target of a symlink is a byte string; bzr treats this as a Unicode string
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in the filesystem encoding (`osutils._fs_enc`).
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So when we say ``bzr add symlink``, this should always add the symlink to
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its containing working tree, and never dereference the symlink.
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However, ``bzr add symlink/file`` shouldn't add ``file`` as a child of
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``symlink``. (Symlinks don't have files underneath them: they may point to
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a directory which contains children, but if the symlink was pointed
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somewhere else those children would be unaffected.) This could either add
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the file in its containing working tree, or fail outright.
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One interesting case for this is ::
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bzr add ~/dev/bug123/a.c
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where ``~/dev`` is actually a symlink to ``/srv/dev/joe/``. In this case
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clearly the user does want us to follow the symlink to open the tree.
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As of bzr2.2, when we open a `WorkingTree`, we typically immediately
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compute its real path and store that as ``.basedir``, but `BzrDir` stores
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its apparent path. (This may not be the best thing.)
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`bzrlib.osutils.dereference_path` does the commonly useful operation of
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resolving the directory part of a path, but leaving the filename
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untouched. In other words ::
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dereference_path('a/b') => 'x/b'
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Relative paths beyond symlinks
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------------------------------
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Another interesting case is when a control directory contains a relative
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path, perhaps from a branch to its master or from a working tree to its
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branch. If it contains ``../`` parts as it typically will, these may have
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different effects depending on whether they're looked up relative to the
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real path or the apparent path given by the user. It may be that some
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users expect different behaviours at different times.
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Resolving the path relative to the real directory makes it somewhat more
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consistent with what you would see by in a shell entering that directory
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and then opening the given name. It may also make things more consistent
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when there are multiple links to the same bzrdir. However it may cause
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problems when using a transport that hides symlinks.
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We could possibly handle this by doing less path arithmetic and asking the
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OS or server to open the path including ``..`` and other relative
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elements, but that might cause other problems. HTTP servers may do their
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own path arithmetic before passing it to the OS.
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Transports that hide symlinks
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-----------------------------
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On local, SFTP and bzr+ssh transports, we can directly see symlinks as
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symlinks. Over HTTP (and FTP?) they're expanded by the server and we
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cannot detect them. This can cause problems when bzr follows relative
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paths because typically we will join the paths, and we may do this
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inconsistently with how the server, which can see the symlinks, would do.
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Symlinks and ChrootTransports
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-----------------------------
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bzr has an internal concept of a `ChrootTransport` that locks access into
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a particular directory. Symlinks should not break out of a chroot jail
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which implies they should be expanded and checked within bzrlib.
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(At least as long as the transport lets us see the symlink; otherwise it
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may not be possible.)
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doc/developers/transports.txt
